In surface micromechanics, sensor elements are delineated out of a mono- or polycrystalline, active silicon surface layer, and rendered freely movable by a sacrificial layer technique, i.e., by the removal of an Sio2 sacrificial layer disposed underneath the structures. For a highly precise pattern delineation of the silicon surface layer, a plasma etching method of the type described in the published German patent document DE 4241045 may be used. To remove the Sio2 sacrificial layer, a hydrofluoric acid vapor etching process may be used, as described, for example, in the published German patent document DE 4317274 and the published German patent document DE 19704454, respectively. A preferred general process for manufacturing surface-micromechanical sensors of this kind is explained, for example, in U.S. Pat. No. 5,756,901. In addition, isotropic silicon sacrificial layer etching technique is described in conjunction with the anisotropic trench technique in the published German patent document DE 4420962 and in the published U.K. patent document GB 2290413, respectively.
Important sensor applications include acceleration sensors and, above all, rotation-rate sensors. Thus, rotation-rate sensors have been designed in accordance with various linear oscillator principles and in accordance with various torsional resonator principles. These rotation-rate sensors are oscillatory structures designed as spring-mass systems, which are excited by electrostatic comb drives into a continuous oscillatory motion of a constant amplitude, and which are evaluated by capacitive sensing structures.
All these structures have in common that the freely movable sensor element is fabricated from one piece of silicon, i.e., the entire system of springs and masses is electrically interconnected and, therefore, is inevitably at the same potential. As a consequence, when working with rotation-rate sensors, for example, there is a harmful feedover, or leakage, of drive voltages into the detector circuit, since, in the absence of a galvanic decoupling within the oscillatory structure, the drive voltages and the detection signals act on one and the same structure.
Often, the decoupling from the drive voltage is accomplished by a downstream spectral filtering, by applying current compensation principles (oppositely directed drive voltages at two comb drives with compensation of the displacement currents), or by a temporal selection process on the basis of switched capacitor principles. In any case, the noise properties of the sensor system deteriorate because of the feedover or due to the countermeasures taken.
In the known methods, a hydrofluoric acid-containing medium in the form of aqueous or vaporous hydrofluoric acid is used for the sacrificial layer etching of the Sio2 layer located underneath the active structures, and the approach which provides for intercalating isolation structures into the functional layer has failed so far, because such isolation structures are destroyed during the sacrificial layer etching. In the same way as the actual Sio2 sacrificial layer, the refill oxide in the isolation trenches is also attacked by the hydrofluoric acid. Particularly when hydrofluoric acid vapor it used, it is virtually impossible to protect functional isolation oxides.